Now that society is well into the information-intensive age in the 21st century, high-performance flat displays for multimedia are becoming of greater interest to facilitate the easy and quick acquisition of extensive information. Now, liquid crystal displays enjoy great predominance over other flat displays, but much effort is now actively being made all over the world to develop new and more economical flat displays with higher performance distinguished from liquid crystal displays. Organic electroluminescent devices, which have lately attracted considerable attention, have advantages of low operation voltage, rapid response speed, high efficiency, and wide viewing angle over liquid crystal displays. In addition, organic electroluminescent displays can readily accommodate the recent trend towards slimness and light weight because the thickness of the modules can be less than 2 mm in total and the plastic substrates as thin as 0.3 mm or less can be used in manufacturing organic electroluminescent displays. Furthermore, the manufacturing cost for organic electroluminescent displays is lower than that for liquid crystal displays.
An organic electroluminescent device typically has a structure of an anode and a cathode with an organic material layer disposed therebetween. On the principle of electroluminescence, such an organic electroluminescent device works by injecting electrons from the cathode and holes from the anode into the organic material layer, combining the electrons and the holes to generate excitons, and transitioning the excitons from excited state to ground state to generate light having certain wavelengths. Now, active research is being made into organic electroluminescent devices in which organic material layers sandwiched between two electrodes have a functionally separated, laminated structure.
For organic luminescent materials in which excitons are generated through combining of holes and electrons and emit light therefrom, single materials may be used in an organic electroluminescent device. Alternatively, the organic luminescent materials may consist of a host material and a guest material which are functionally different. Here, the host material functions to generate excitons with received holes and electrons and then to transfer the energy of excitons to the guest material, and the guest material functions to emit light by way of forming excitons with the transferred energy.
Host materials or guest materials alone can emit light in organic electroluminescent devices. However, when used alone, luminous efficiency and brightness are low, and intermolecular self-packing sometimes causes not only to change its intrinsic property, but also its excimer's emission, such as inclination from a pure intrinsic light color toward white color. These problems can be avoided by using a host material doped with a small amount of a guest material.
In a luminescent material, the excitons, that are generated when electrons and holes are coupled with each other, are divided into fluorescent excitons that emit light through singlet-singlet transition and phosphorescent exitons that emit light through triplet-singlet transition. Here, a material from which light is generated via fluorescent excitons is called a fluorescent material and a material from which light is generated via phosphorescent excitons is called a phosphorescent material.
It is well known that Fluorescent excitons and phosphorescent excitons are formed at a probability ratio of 1:3 in a luminescent material. Thus, an organic electroluminescent device employing a phosphorescent material as a luminescent material is preferable in terms of luminous efficiency. In addition, when a host material is doped with a guest material, a guest material is preferably a phosphorescent material in terms of luminous efficiency. In this case, because the energy of a host material is not transferred only through light to a guest material, the host material may be a fluorescent material. However, the energy band gap of a host material that is used in combination with a phosphorescent guest material must be far larger than that of a host material that is used in combination with a fluorescent guest material. The reason is as follows.
The energy of triplet excitons is usually known to be lower than that of singlet excitons. Thus, when the energy of excitons generated in a host material is transferred into a guest material, the triplet excitons of the host material must have larger energy than do the singlet excitons of the guest material. By the way, a phosphorescent guest material, especially a blue light-emitting phosphorescent guest material, has a far larger energy gap than a fluorescent guest material. Accordingly, in order to transfer the energy of triplet excitons generated in a host material to a phosphorescent guest material, the energy band gap of the host material must be far larger than that of a host material that is used in combination with a fluorescent guest material.